It has been recognized for some time that detection and measurement of gamma rays in a borehole can be used to characterize the underground formation through which the borehole passes. Measurement of naturally ocurring gamma rays can provide some information but it has been found that more extensive information can be obtained by irradiating the formation with high energy neutrons. The neutrons interact with formation nuclei to cause gamma ray production by way of inelastic neutron scattering and neutron capture (neutron absorption). In inelastic neutron scattering, the neutron bounces off the nucleus but excites it into quickly giving off what are called inelastic gamma rays. The measurement of gamma ray energies from inelastic neutron scattering can be used to yield the relative concentrations of carbon and oxygen (C/O logging) which can be used to determine the water saturation of the formation. In neutron absorption, the nucleus absorbs the neutron and becomes excited, typically accompanied by emission of capture gamma rays. Neutron absoption, or neutron capture is most common after a neutron has been slowed by elastic and inelastic interactions to thermal energies of about 0.025 eV. The measurement of capture gamma ray energies can be used to estimate the abundances of certain elements in the formation, typically silicon, calcium, chlorine, hydrogen, sulfur, iron, titanium and gadolinium. Examples of tools and methods for gamma ray logging are shown in U.S. Pat. Nos. 4,390,783, 4,430,567, 4,464,569, 4,507,554, 4,883,956, 5,097,123 and 5,237,594.
Tools for gamma ray logging comprise a source of neutrons, typically a pulsed accelerator source such as a D-T source producing neutrons having an energy of about 14 MeV, and a gamma ray detector such as a scintillator and photomultiplier tube combination. The scintillator is generally a crystal such as thallium-doped sodium iodide although other scintillators such as bismuth germanate, cerium-doped gadolinium oxyorthosilicate or cerium-doped lutetium oxyorthosilicate might also be used.
The determination of formation porosity can also be performed by irradiating the formation with neutrons and making measurements in the borehole. However, in this case it is normal to detect the neutron returning to the borehole after interaction with formation nuclei by means of a neutron detector such as a .sup.3 He proportional counter. The neutron source for porosity determination can be the same as that used for gamma ray logging. Examples of tools and methods for porosity determination by neutron logging are given in U.S. Pat. No. 4,760,252.
The effect of neutrons interacting with the elements contained in the tool itself has been observed in C/O logging tools, see for example The Log Analyst, November-December 1993, pp 11-19. It was observed that the magnitude of the tool contribution was dependent on the gap between the tool and the borehole wall and on the presence of water. This background has been found to be variable and has caused problems in interpreting measurements made with gamma ray detectors in boreholes.
It is an object of the invention to provide a method of using the tool contribution to determine the porosity of the formation under investigation.